1. Field of the Invention
The present invention relates to a package of polyurethane elastic yarn to be used for heat bonding. More particularly, the present invention relates to a package of polyurethane elastic yarn to be used to form the waist and leg parts of disposable diaper which need elastic properties. The polyurethane elastic yarn exhibits good bonding properties when disposable diapers undergo bonding such as hot-melt bonding, fusion bonding, and ultrasonic bonding during their production. The package keeps its good appearance in shape without slipping down of yarn layer and also permits good unwinding without mutual sticking of the polyurethane elastic yarn.
2. Description of the Prior Art
Owing to its good elastic properties, polyurethane elastic yarn is used in various fields where extensibility and fittability are required. Polyurethane elastic yarn is usually supplied in the form of package and hence it is given a finishing agent so that it does not stick mutually in the package and it is unwound without unnecessary resistance.
In addition, in the case where polyurethane elastic yarn is used continuously it is necessary to replace packages frequently. The less the amount of yarn in the package, the greater the frequency of replacement. In order to reduce the frequency of package replacement, it has become common practice to make packages larger. In the case of large packages, application of finishing agent is indispensable to prevent yarn sticking which occur particularly in the core layer of the package.
Unfortunately, a polyurethane elastic fiber treated with a finishing agent has the disadvantage of being very poor in bonding properties due to the finishing agent present on it. This is true in the case where the gather of a disposable diaper is formed by adhesion or heat bonding with another material.
One way to solve this problem is disclosed in Japanese Patent Publication No. 50429/1993. According to this disclosure, a polyurethane elastic yarn is given a finishing agent composed mainly of silicone oil in an amount less than 2 wt % and then wound up such that its apparent elongation in the package is within a certain range for the amount of the finishing agent applied. Such polyurethane elastic yarn exhibits good adhesion properties with adhesive in the production of disposable diapers. The advantage of this method is that the polyurethane elastic fiber is not so poor in adhesion properties because of the smaller amount less than 2 wt % of the finishing agent used. On the other hand, this method has the disadvantage that the package of the polyurethane elastic yarn gives a poor appearance in shape because it is wound up with a lower tension than usual so as to prevent mutual sticking of yarn. This is true in the case where the apparent elongation is low even though it is within the certain range for the amount of the finishing agent used. Conversely, with a great apparent elongation, the polyurethane elastic yarn is not unwound smoothly due to mutual sticking of the yarn in the package. Thus, it is substantially difficult to obtain the package of polyurethane elastic yarn which appearance has a good in shape and is unwound smoothly by simply reducing the amount of finishing agent and defining the apparent elongation.
Japanese Patent Laid-open No. 152264/1998 discloses a package of elastic yarn which is given 2-5 wt % of a finishing agent, for example silicone oil, having a surface tension of 25-30 dyn/cm. It is claimed that this package is unwound smoothly and keeps its adhesion properties when used to make a disposable diaper. However, it has been found that the finishing agent having a surface tension of 25-30 dyn/cm as specified above does not provide satisfactory adhesion properties even when it is used in an amount of 2-5 wt %.